Thermoelectric conversion is an energy conversion method in which the Seebeck effect is utilized to impart the temperature difference between both ends of a thermoelectric conversion material, thereby generating the potential difference to conduct electricity generation. In such thermoelectric generation, electricity is obtained by only disposing one end of the thermoelectric conversion material on a hot section produced by waste heat and disposing other end thereof in the air (room temperature), and connecting a conducting wire to each of both the ends. Accordingly, any movable appliances such as a motor and a turbine necessary for common electricity generation are not required at all. Therefore, there are the following advantages: the cost of electricity generation is low; any gas by burning or the like is not emitted; and electricity generation can be continuously performed until the thermoelectric conversion material is degraded.
As an oxide having n-type thermoelectric conversion characteristics, there have been proposed a calcium-manganese composite oxide of CaMnO3, such a calcium-manganese composite oxide in which Ca or Mn is partially replaced with a proper element, and the like, and the calcium-manganese composite oxide is expected to be put in practical use as an n-type oxide thermoelectric conversion material because of exhibiting favorable conductivity even in air at high temperatures and having a Seebeck coefficient of more than 100 μV/K (for example, Patent Literature 1).
On the other hand, as an oxide having p-type thermoelectric conversion characteristics, there have also been proposed a CoO2-based layered oxide such as calcium cobaltite (Ca3Co4O9), a CoO2-based layered oxide in which Ca and/or Co of the calcium cobaltite are/is partially replaced with a proper element, and the like (see, for example, Patent Literatures 2 to 3).
A thermoelectric conversion material in which a CoO2-based layered oxide is used is prepared so that a plate crystal of the CoO2-based layered oxide is used and the crystal plane is oriented in one direction in order to exhibit excellent thermoelectric characteristics.
Patent Literature 4 below has proposed a thermoelectric conversion element in which a CoO2-based layered oxide is used as a p-type thermoelectric conversion material, and has disclosed, as a method for preparing the thermoelectric conversion material, a method including subjecting a plate crystal of the CoO2-based layered oxide to pressure molding, followed by sintering by hot pressing under pressure, so-called a pressure sintering method.
While the pressure sintering method in Patent Literature 4 provides a thermoelectric conversion material in which a plate crystal of the CoO2-based layered oxide is oriented in the crystal plane direction and which is excellent in orientation, the pressure sintering method has difficulty in preparing a sintered product in a large amount at the same time, and is not industrially effective.
As a method for producing a thermoelectric conversion material including a CoO2-based layered oxide in an industrially effective manner, Patent Literature 5 has proposed a method including subjecting a plate powder made of a cobalt compound such as Co3O4 or Co (OH)2, a calcium compound and a fluorine compound to molding according to a doctor blade method, a press molding method, a rolling method, an extrusion method, or the like, followed by sintering at normal pressure.